<!doctype html>
<html>

<head>
    <meta charset="UTF-8">
    <title>嘿嘿嘿</title>
    <meta name="viewport"
          content="width=device-width, initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no, minimal-ui, viewport-fit=cover">

</head>
<style type="text/css">
    body {
        background: #f5f5f5;
    }

    .father-box {
        /*position: relative;*/
        width: 150px;
        height: 150px;
        background-color: black;
        margin: 20px;
    }

    .father-box .son-box {
        width: 50px;
        height: 50px;
        background-color: greenyellow;
        margin-top: 20px;
    }

    .brother-two {
        width: 150px;
        height: 20px;
        background-color: gold;
        margin: 20px;
    }
</style>
<body>
<div class="father-box brother">
    <div class="son-box">

    </div>
</div>
<div class="brother brother-two">
    黄色的是20px的高度
</div>

</body>

</html>
